Electrolysis System For The Electrochemical Utilization Of Carbon Dioxide

ABSTRACT

The present disclosure relates to electrolysis. Teachings thereof may be embodied in a reduction method and/or an electrolysis system for electrochemical carbon dioxide utilization. For example, an electrolysis system for carbon dioxide utilization may include: an electrolysis cell with an anode in an anode space, a cathode in a cathode space, and a membrane; a first feed for carbon dioxide into the cathode space, configured to bring the carbon dioxide into contact with the cathode; a proton donor unit; and a second feed for protons configured to bring the protons into the cathode space from the proton donor unit into contact with the cathode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application of InternationalApplication No. PCT/EP2016/061177 filed May 19, 2016, which designatesthe United States of America, and claims priority to DE Application No.10 2015 209 509.6 filed May 22, 2015, the contents of which are herebyincorporated by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to electrolysis. Teachings thereof may beembodied in a reduction method and/or an electrolysis system forelectrochemical carbon dioxide utilization. Typically, carbon dioxide isintroduced into an electrolysis cell and reduced at a cathode.

BACKGROUND

At present, about 80% of the global energy requirement is covered by thecombustion of fossil fuels, the combustion processes of which causeglobal emission of about 34 000 million tons of carbon dioxide into theatmosphere per annum. This release into the atmosphere includes themajority of carbon dioxide released, which can be up to 50 000 tons perday in the case of a brown coal power plant, for example. Carbon dioxideis one of the gases known as greenhouse gases. Since carbon dioxide hasa relatively very low thermodynamic level, it can be converted to usefulproducts only with difficulty, which has left the actual reutilizationof carbon dioxide in the realm of theory or in the academic field todate.

Natural carbon dioxide degradation proceeds, for example, viaphotosynthesis. This includes conversion of carbon dioxide tocarbohydrates in a process subdivided into many component steps overtime and, at the molecular level, in terms of space. As such, thisprocess cannot easily be adapted to the industrial scale. No copy of thenatural photosynthesis process with photocatalysis on the industrialscale to date has had adequate efficiency to be implemented.

One alternative is the electrochemical reduction of carbon dioxide.Systematic studies of the electrochemical reduction of carbon dioxideare still a relatively new field of development. Only in the last fewyears have there been efforts to develop an electrochemical system thatcan reduce an acceptable amount of carbon dioxide. Research on thelaboratory scale has shown that electrolysis of carbon dioxide may beaccomplished using metals as catalysts. The publication “ElectrochemicalCO2 reduction on metal electrodes” by Y. Hori, published in: C. Vayenas,et al. (eds.), Modern Aspects of Electrochemistry, Springer, New York,2008, p. 89-189, discloses Faraday efficiencies at different metalcathodes; see table 1. While carbon dioxide is reduced almostexclusively to carbon monoxide at silver, gold, zinc, palladium andgallium cathodes, for example, a multitude of hydrocarbons form asreaction products at a copper cathode.

For example, at a silver cathode, predominantly carbon monoxide and alittle hydrogen would form. Possible reactions at anode and cathode canbe represented by the following reaction equations:

Cathode: 2CO₂+4e ⁻+2H₂O→2CO+4OH⁻

2CO₂+12e ⁻+8H₂O→C₂H₄+4OH⁻

CO₂+8e ⁻+6H₂O→CH₄+8OH⁻

Anode: 2H₂O→O₂+4H⁺+4e ⁻

Or alternatively, if a chloride-containing electrolyte is present:

2Cl⁻→Cl₂+2e ⁻

Of particular economic interest, for example, is the electrochemicalproduction of carbon monoxide, methane or ethene. These arehigher-energy products than carbon dioxide.

TABLE 1 Electrode CH₄ C₂H₄ C₂H₅OH C₃H₇OH CO HCOO⁻ H₂ Total Cu 33.3 25.55.7 3.0 1.3 9.4 20.5 103.5 Au 0.0 0.0 0.0 0.0 87.1 0.7 10.2 98.0 Ag 0.00.0 0.0 0.0 81.5 0.8 12.4 94.6 Zn 0.0 0.0 0.0 0.0 79.4 6.1 9.9 95.4 Pd2.9 0.0 0.0 0.0 28.3 2.8 26.2 60.2 Ga 0.0 0.0 0.0 0.0 23.2 0.0 79.0102.0 Pb 0.0 0.0 0.0 0.0 0.0 97.4 5.0 102.4 Hg 0.0 0.0 0.0 0.0 0.0 99.50.0 99.5 In 0.0 0.0 0.0 0.0 2.1 94.9 3.3 100.3 Sn 0.0 0.0 0.0 0.0 7.188.4 4.6 100.1 Cd 1.3 0.0 0.0 0.0 13.9 78.4 9.4 103.0 Tl 0.0 0.0 0.0 0.00.0 95.1 6.2 101.3 Ni 1.8 0.1 0.0 0.0 0.0 1.4 88.9 92.4 Fe 0.0 0.0 0.00.0 0.0 0.0 94.8 94.8 Pt 0.0 0.0 0.0 0.0 0.0 0.1 95.7 95.8 Ti 0.0 0.00.0 0.0 0.0 0.0 99.7 99.7

The table gives Faraday efficiencies [%] of products that form in thecarbon dioxide reduction at various metal electrodes. The valuesreported apply to a 0.1 M potassium hydrogen-carbonate solution aselectrolyte and current densities below 10 mA/cm².

In the electrochemical conversion of matter of carbon dioxide to ahigher-energy product, the increase in the current density and hence theincrease in the conversion of matter is of interest. It is not easy toassure a high current density or to increase it even further, since, inthe methods known to date and the electrolysis systems used, it isnecessary to take account of macrokinetic effects, for instance masstransfer limitations in the immediate proximity of the solid-liquidinterface from the electrolyte to the electrode. The carbon dioxide isreduced at the catalytically active cathode surface.

To date, the problem of mass transfer limitation has been countered byusing gas diffusion electrodes, which can have a process-intensifyingeffect and have indeed already made existing electrochemical methodseconomically viable and competitive. Beyond this approach, no furtherincrease in conversion of matter has been possible to date.

Electrolysis cells suitable for electrochemical reduction of carbondioxide typically consist of an anode space and a cathode space. FIGS. 2to 4 show examples of cell arrangements in a schematic diagram. Theconstruction with a gas diffusion electrode is shown, for example, inFIG. 3. In this execution of an electrolysis cell, the carbon dioxide isintroduced through a porous cathode directly from the cathode surfaceinto the cathode space.

The existing methods of carbon dioxide reduction include conversion ofcarbon dioxide in physically dissolved or gaseous form in the reactionspace. None of the known approaches to a solution for carbon dioxidereduction makes use of the chemically bound carbon dioxide content inthe electrolysis system: the total molar amount of carbon dioxidepresent in the electrolysis system is composed of a chemical componentand a physical component. Whether the carbon dioxide is in chemicallybound or physically dissolved form in the electrolyte depends on variousfactors, for example the pH, the temperature, the electrolyteconcentration or the partial pressure of the carbon dioxide. Both carbondioxide components are involved in an equilibrium relationship. In thesystem of carbon dioxide in aqueous carbonate or hydrogencarbonatesolution, this equilibrium relationship can be described by thefollowing chemical equation:

CO₂+H₂O

H₂CO₂

HCO₃ ⁻+H⁺

CO₃ ²⁻+H⁺  Eq. 1

as carbonic acid (H₂CO₃) or as carbonate (CO₃ ²⁻), for example aspotassium carbonate or potassium hydrogencarbonate, as occurs in asystem for potash scrubbing, the carbon dioxide is in chemically boundform. But carbon dioxide can also be in gaseous or physically dissolvedform. The physical dissolution process also proceeds until establishmentof a dissolution equilibrium which, under the assumption of Henry's law,is likewise temperature-, concentration- and pressure-dependent:

χ_(i)·H_(ij)═P  Eq. 2

In this equation, χ_(i) represents the molar amount and is less than0.01. P represents the pressure and is less than 2 bar. H_(ij)represents the Henry constant.

It has been shown, for example in the publication “CO2-reduction,catalyzed by metal electrodes” by Y. Hori, published in: Handbook ofFuel Cells—Fundamentals, Technology and Applications, W. Vielstich etal. (eds.), John Wiley & Sons, Ltd., 2010, p. 2 and FIG. 1, thatexclusively physically dissolved carbon dioxide is amenable to anelectrochemical conversion. If one wished to increase this physicallydissolved carbon dioxide according to Henry's law, for example via anelevated partial carbon dioxide pressure P, the result of this would be,according to the chemical equilibrium reaction eq. 1, that a highproportion reacts to give carbonates or hydrogencarbonates and hence theactual carbon dioxide concentration in solution is reduced again.

In spite of the increase in the dissolved carbon dioxide content, withincreasing conversion of matter, there is a limitation directly at theelectrode surface resulting from the mass transfer from the cathodespace to the cathode interface. In such a case, there can also be anincrease in unwanted hydrogen production at the reaction surface as acompeting process to carbon dioxide reduction. The formation of hydrogenat the cathode surface in turn leads automatically to a decrease inproduct selectivity.

FIG. 1 shows, by way of illustration of the dependence of theconcentration and pH parameters, an example of a Hagg diagram of a 0.05molar solution of carbon dioxide. Within a moderate pH range, carbondioxide and salts thereof are present alongside one another. Whilecarbon dioxide (CO₂) is preferentially in the form of carbonate (CO₃ ²⁻)in the strongly basic range and preferentially in the form ofhydrogencarbonate (HCO₃ ⁻) in the moderate pH range, thehydrogencarbonate ions are driven out of the solution in the form ofcarbon dioxide at low pH values in the acidic medium. According to theHagg diagram and equations 1 and 2, the carbon dioxide concentration ina hydrogencarbonate-containing electrolyte can be very low in spite of ahigh hydrogencarbonate concentration in the range from 0.1 mol/L to wellabove 1 mol/L up to the solubility limit of the corresponding salt.

It can also be illustrated once again by this diagram that, according tothe pH and concentration, a high proportion of carbon dioxide is inchemically bound form and hence is unavailable for the electrochemicalutilization.

SUMMARY

The teachings of the present disclosure may be embodied in an improvedsolution for electrochemical carbon dioxide utilization which avoidsthese disadvantages described above. More particularly, the solutionproposed is to enable particularly effective conversion of carbondioxide.

For example, an electrolysis system for carbon dioxide utilization, maycomprise: an electrolysis cell (6 . . . 9) having an anode (A) in ananode space (AR), a cathode (K) in a cathode space (KR), and at leastone membrane (M₁), wherein the cathode space (KR) has a first feed forcarbon dioxide (CO₂) and is configured to bring the carbon dioxide (CO₂)fed in into contact with the cathode (K), characterized in that theelectrolysis system comprises a proton donor unit and the cathode space(KR) is connected to the proton donor unit via a second feed for protons(H⁺) which is configured to bring the protons (H⁺) fed into the cathodespace (KR) into contact with the cathode (K).

In some embodiments, the proton donor unit comprises a proton reservoir(PR) and a proton-permeable membrane (M2) which functions as a secondfeed to the cathode space (KR) for the protons (H⁺).

In some embodiments, the proton reservoir (PR) is an acid reservoir,especially comprising a Brønsted acid (HX).

In some embodiments, the proton-permeable membrane (M2) includessulfonated polytetrafluoroethylene.

In some embodiments, the cathode space (KR) includes a catholyte/carbondioxide mixture, wherein the catholyte comprises carbonate (CO₃ ²⁻)and/or hydrogencarbonate anions (HCO₃ ⁻) and/or dihydrogen carbonate(H₂CO₃).

In some embodiments, the anode space (AR) functions as a protonreservoir (PR).

In some embodiments, there is a first membrane and a second membrane(M1, M2), wherein the first membrane (M1) is arranged between the anode(A) and cathode (K), the second membrane (M2) is arranged between thecathode (K) and proton reservoir (P), and at least the second membrane(M2) is proton-permeable.

In some embodiments, the cathode space (KR) is executed as a catholytegap (KS) that extends along the cathode (K) and has an extent at rightangles to the surface area of the cathode of not more than 5 mm.

In some embodiments, the cathode space (KR) is executed as a catholytegap (KS) which separates the cathode (K) and membrane (M1, M2), whereincathode (K) and membrane (M1) are arranged at a distance of not morethan 5 mm from one another.

In some embodiments, the cathode space (KR) comprises two catholyte gaps(KS) arranged on either side of the cathode (K), each of which isbounded by a membrane (M1, M2), wherein the cathode (K) and membranes(M1, M2) are each independently arranged at a maximum distance of 5 mmfrom one another.

In some embodiments, there is a proton donor cathode (PSK) comprisingthe proton donor unit and a proton-permeable cathode (KP) integratedtherein.

Some embodiments may include a reduction method for carbon dioxideutilization by means of an electrolysis system as described above, inwhich a catholyte/carbon dioxide mixture is introduced into a cathodespace (KR) and brought into contact with a cathode (K), and in whichlocal lowering of the pH of the catholyte/carbon dioxide mixture isundertaken in the cathode space (KR) by providing additional protons(H⁺).

In some embodiments, the local lowering of the pH of thecatholyte/carbon dioxide mixture is undertaken at the liquid/solid phaseinterface from the catholyte/carbon dioxide mixture to the cathode (K)by providing the additional protons (H⁺) via the proton-permeablemembrane (M) or via the proton-permeable cathode (K) at the liquid/solidphase interface from the catholyte/carbon dioxide mixture to the cathode(K).

In some embodiments, the protons (H⁺) are taken from a proton reservoir(PR), especially an acid reservoir which especially comprises a Brønstedacid (HX), e.g. sulfuric acid (H₂SO₄), phosphoric acid (H₃PO₄) or nitricacid (HNO₃), hydrochloric acid (HCl), or organic acids such as aceticacid and formic acid.

In some embodiments, the catholyte includes carbonate (CO₃ ²⁻) and/orhydrogencarbonate anions (HCO₃ ⁻).

BRIEF DESCRIPTION OF THE DRAWINGS

Examples and embodiments of the teachings of the present disclosure aredescribed by way of example with reference to FIGS. 1 to 9 of theappended drawings:

FIG. 1 shows a Hagg diagram for a 0.05 molar carbon dioxide solution,

FIG. 2 shows a schematic diagram of a two-chamber setup of anelectrolysis cell, according to teachings of the present disclosure;

FIG. 3 shows a schematic diagram of a three-chamber setup of anelectrolysis cell, according to teachings of the present disclosure;

FIG. 4 shows a schematic diagram of a PEM setup of an electrolysis cell,according to teachings of the present disclosure;

FIG. 5 shows an electrolysis cell in a two-chamber setup and thecharacteristic rise in pH toward the cathode, according to teachings ofthe present disclosure;

FIG. 6 shows a schematic diagram of a cell arrangement with anadditional acid reservoir and porous cathode, according to teachings ofthe present disclosure;

FIG. 7 shows a cell arrangement with an additional acid reservoir andtwo catholyte gaps, according to teachings of the present disclosure;

FIG. 8 shows a schematic diagram of a further example of a cellarrangement with an additional acid reservoir and porous cathode,according to teachings of the present disclosure; and

FIG. 9 shows a schematic diagram of a further embodiment of a cellarrangement with an additional acid reservoir and electrolyte gaps,according to teachings of the present disclosure.

DETAILED DESCRIPTION

In some embodiments, an electrolysis system for carbon dioxideutilization comprises an electrolysis cell having an anode in an anodespace, a cathode in a cathode space and at least one membrane, whereinthe cathode space has a first feed for carbon dioxide and is configuredto bring the carbon dioxide fed in into contact with the cathode.“Membrane” is understood here to mean a mechanically separating layer,for example a diaphragm, which separates at least the electrolysisproducts formed in the anode space and cathode space from one another.This can also be referred to as a separator membrane or separatinglayer. Since the electrolysis products can also be gaseous substances,some embodiments include a membrane having a high bubble point of 10mbar or higher. The “bubble point” is a defining parameter for themembrane used, which describes the pressure difference AP between thetwo sides of the membrane from which gas flow through the membrane wouldset in.

In some embodiments, carbon dioxide in chemically bound form, forexample as carbonate or hydrogencarbonate in the electrolyte, can beintroduced into the cathode space via the first feed for carbon dioxide,or else carbon dioxide gas can be introduced into the cathode space viathe first feed separately from the electrolyte or physically dissolvedcarbon dioxide in an electrolyte. More particularly, the feed may be theelectrolyte and reactant inlet. Even when the carbon dioxide enters thecathode space in gaseous or dissolved form, a proportion thereof entersinto a chemical compound with substances present in the electrolyteaccording to the equilibrium reactions described above, especially whenthe pH is basic.

In some embodiments, the electrolysis system comprises a proton donorunit and the cathode space is connected to the proton donor unit via asecond feed for protons. The second feed for protons is configured suchthat the protons are brought into contact with the cathode surface inthe cathode space. The proton donor unit is defined here in that freeprotons, e.g. hydrogen cations, are provided. Hydrogen (H₂) or otherhydrogen compounds are not protons for the purposes of the proton donorunit of the invention.

In some embodiments, by means of the proton donor unit, local loweringof the pH is possible in the electrolysis system, which promotes theformation of physically dissolved carbon dioxide at the reactioninterface of the cathode and significantly increases the conversion ofmatter.

In some embodiments, the electrolysis system comprises a proton donorunit having a proton reservoir and a proton-permeable membrane. Theproton-permeable membrane functions here as a second feed to the cathodespace for the protons. While the proton reservoir offers continuousreplenishment of protons, the proton-permeable membrane serves to assurepure ion flow or proton flow to the cathode space and simultaneously toretain other molecules, liquids, or gases. The proton-permeable membranemay include sulfonated polytetrafluoroethylene. In some embodiments, acation exchange membrane comprises a proton-permeable membrane.

In some embodiments, the electrolysis system has an acid reservoir asproton reservoir which especially comprises a Brønsted acid. A Brønstedacid is, for example, sulfuric acid, phosphoric acid, nitric acid,hydrochloric acid, or various organic acids, for example acetic acid orformic acid. The definition of an acid according to Brønsted describesacids as so-called proton donors, particles that can release protons,e.g., positively charged hydrogen ions. According to the definition ofthe pKa according to equation 1, Brønsted acids may have a pKacorrespondingly smaller than the pKa of aqueous carbonate,hydrogencarbonate, or dihydrogen carbonate solution. “Smaller” in thiscase means that the acid is stronger. Using an acid reservoir mayprovide a relatively continuous proton source which is not reliant on anadditional external energy input.

In some embodiments, the electrolysis system has a secondproton-permeable membrane comprising sulfonated polytetrafluoroethylene.The proton-permeable membrane used may include a Nafion membrane. Thismembrane may include, for example, a multilayer or porous form. Thefirst membrane used, e.g. the separator membrane, may likewise be aproton-permeable membrane, like that of the proton donor unit.

In some embodiments, the cathode space of the electrolysis systemcomprises a catholyte/carbon dioxide mixture, wherein the catholytecomprises carbonate and/or hydrogencarbonate anions. In addition, thecatholyte in the cathode space of the electrolysis system may includealkali metal and/or ammonium ions (NH₄ ⁺). Alkali metals refer to thechemical elements lithium, sodium, potassium, rubidium, cesium, andfrancium from the first main group of the Periodic Table. The carbonate-and/or hydrogencarbonate-containing electrolyte has the advantage ofincluding chemically bound carbon dioxide. In some embodiments, carbondioxide can be introduced into the cathode space in dissolved or gaseousform. The pH of the catholyte in the cathode space preferably has avalue between 4 and 14.

In some embodiments, the electrolysis system comprises an anode spacewhich functions as proton reservoir. It is possible here, for example,to use an electrolysis system in which a single proton-permeablemembrane simultaneously fulfills the function of separating cathodespace and anode space and the function of admitting protons into thecathode space. In some embodiments, the anode space which functions asthe proton reservoir is connected to the cathode space by the membraneand an anode in porous form. Further alternatives will be apparent fromthe embodiments that are still to follow that have two protonreservoirs, for example including connected proton reservoirs. It is notnecessary for the proton reservoirs to be connected, since protons canalso be produced again at the anode, which depends on the electrolyteconcentration. The concentration has to be correspondingly high for therelease of carbon dioxide.

In some embodiments, the electrolysis system has a first membrane and asecond membrane, wherein the first membrane is arranged between theanode and cathode as separator membrane, the second membrane is arrangedbetween the cathode and proton reservoir, and at least this secondmembrane is proton-permeable. This arrangement of the electrolysissystem provides the connection of the proton reservoir via theproton-permeable membrane to the cathode and ensures that the protonsare supplied directly to the reaction surface of the cathode. For thispurpose, the cathode is may have a porous form and is in direct,two-dimensional contact with the proton-permeable membrane adjoining theproton reservoir. In this setup, for example, anolyte, catholyte andproton source, for example an acid or acid mixture, can be chosenseparately from one another and preferably matched to one another.

In some embodiments, the cathode space of the electrolysis system is inthe form of a catholyte gap that extends along the cathode and has awidth, an extent at right angles to the surface area of the cathode, ofnot more than 5 mm. A catholyte gap is accordingly understood to mean athin hollow space in two-dimensional form between the cathode and amembrane. The membrane bounds the catholyte gap, for example, from theproton reservoir or from the anode space or the anode. In the case of agreater gap width than 5 mm, the pH gradient described again plays anon-negligible role in the cathode space. In some embodiments, thecathode space in the electrolysis system includes a catholyte gap whichseparates the cathode and proton-permeable membrane or the cathode andthe first membrane, and these are each arranged at a distance of notmore than 5 mm from one another.

In some embodiments, the cathode space may also comprise two catholytegaps arranged on either side of the cathode, each of which is bounded bya membrane, wherein the cathode and membranes are each independentlyarranged at a maximum distance of 5 mm from one another. In this way,electrolysis products can be generated on both sides of the cathode.These embodiments have the benefit that it is possible to use a solidcathode, for example, a cathode sheet, meaning that the cathode is notin porous form. A solid cathode of this kind may have a nanostructuredsurface. In the case of a solid cathode, both membranes are inproton-permeable form to correspondingly assure proton access.

In some embodiments, there is relatively a small distance betweenproton-conducting membrane and cathode, or between separator membraneand cathode in the case of the integrated proton donor cathode: thisdistance is typically between 0 and 5 mm, e.g. between 0.1 and 2 mm. Adistance of 0 mm would correspond to a polymer electrolyte membrane(half-)cell.

In some embodiments, the electrolysis system comprises a proton donorcathode comprising the proton donor unit and a proton-permeable cathodeintegrated therein. In this case, the cathode is porous, for example, inthe form of a perforated sheet electrode, of a sieve electrode, of alattice electrode, mesh electrode or weave electrode or, like a gasdiffusion electrode, composed of compressed nano- to microparticles,optionally with additional membrane plies. The proton-permeable cathodehere may be bonded directly to, for example applied to, theproton-permeable membrane, or vice versa, and hence forms part of thesecond feed to the cathode space for the protons. In this configuration,the protons enter the cathode space from the proton reservoir over theentire cathode area, exactly at the point in the cathode space, and thephase interface between cathode surface and catholyte, at which they areto release the carbon dioxide from the catholyte. According to theirfunction and arrangement, this variant was referred to as proton donorcathode.

In some embodiments, the proton-donating membrane of the proton donorunit can be arranged in the immediate proximity of the cathode;secondly, the cathode can be integrated into the proton donor unit withthe proton-donating membrane.

Some embodiments may include a reduction method for carbon dioxideutilization by means of an electrolysis system according to any of theembodiments described. In these embodiments, a catholyte/carbon dioxidemixture may be introduced into a cathode space and brought into contactwith a cathode, and local lowering of the pH of the catholyte/carbondioxide mixture is undertaken in the cathode space by providingadditional protons. The additional protons serve to produce reduciblecarbon dioxide which is in physically dissolved or gaseous form but isno longer chemically bound, this carbon dioxide being generated orreleased directly at the cathode reaction interface. This local increasein carbon dioxide concentration significantly increases the conversionthereof.

In some embodiments, in the reduction method, the local lowering of thepH of the catholyte/carbon dioxide mixture is undertaken at theliquid/solid phase interface from the catholyte/carbon dioxide mixtureto the cathode by providing the additional protons via theproton-permeable membrane or via the proton-permeable cathode at theliquid/solid phase interface from the catholyte/carbon dioxide mixtureto the cathode. This brings about in situ carbon dioxide generation inthe phase interface region from the hydrogencarbonate or carbonateanions present in the electrolyte.

In some embodiments, in the reduction method, protons are taken from aproton reservoir, especially an acid reservoir which especiallycomprises a Brønsted acid, e.g. sulfuric acid, phosphoric acid, and/ornitric acid, hydrochloric acid or an organic acid such as acetic acidand formic acid.

In some embodiments, the catholyte includes carbonate and/orhydrogencarbonate anions and/or dihydrogen carbonate. In addition, thecatholyte may include alkali metal and/or ammonium ions. In someembodiments, the catholyte includes sulfate and/or hydrogensulfate ions,phosphate, hydrogenphosphate, and/or dihydrogenphosphate ions.

In some embodiments, the pH of the catholyte is within a range between 4and 14.

In the working example, by virtue of a conductive porous catalystcathode integrated into the proton donor unit in such a way that theprotons are introduced into the cathode space via the proton-conductingmembrane and directly thereafter through the cathode itself, theproton-conducting membrane can be backflushed, for example, by an acid.The acid strength may be adjusted such that the amount of carbon dioxidedriven out of the catholyte is specifically as much as can be reduced atthe cathode at a given current density. It is possible in this way toensure that the product formed or the product mixture is very low incarbon dioxide.

The cathode itself may have a large surface area. In the case of apolymer electrolyte membrane (PEM) setup, the cathode itself may be inporous form, which likewise means an increase or maximization in thereactive surface area. The cathode used may include an RVC (reticulatedvitreous carbon) electrode. This may be permeable to the electrolyteitself and, by contrast to a gas diffusion electrode, has no hydrophobicconstituents. This variant may be suitable with an electrolysis cell asshown in FIG. 4. In some embodiments, the cathode used comprises asilver gas diffusion electrode. In some embodiments, this can also beexecuted with zero carbon content. A silver gas diffusion electrode usedcomprises, for example, silver (Ag), silver oxide (Ag₂O) and/orpolytetrafluoroethylene (PTFE, e.g. Teflon).

Some embodiments may enable the conversion of the carbon dioxide contentchemically bound in carbonates and hydrogencarbonates to physicallydissolved carbon dioxide or carbon dioxide gas, which constitutes thedesired starting components for the electrochemical carbon dioxidereduction. What are thus described are a method and a system that enablehigh carbon dioxide conversions with current densities >>100 mA/cm²,without requiring an electrode with separate gas supply as cathode. Agas diffusion electrode as used to date could be introduced as anadditional component in some embodiments.

The phase interface layer between the proton-conducting membrane of theproton donor unit and the catholyte or the phase interface layer betweenthe cathode surface and the catholyte effectively itself serves as acarbon dioxide source. In this phase interface layer, a local change inpH occurs as a result of the migrating protons. The equilibrium reaction1 is then affected in such a way that finely divided carbon dioxide gasbubbles arise at the membrane surface or cathode surface throughbreakdown of carbonate in the acidic medium.

In some embodiments, the locally acidic pH is also determined by theBrønsted-acidic surface of the proton-conducting membrane or by theacidic sulfonic acid groups that exist at the cathode surface. Thesulfonic acid groups come from the sulfonated polytetrafluoroethylene inthe membrane. The latter comprises, for example, Nafion-Teflonadditionally containing a directly coupled sulfonic acid group. Inwater, this polymer swells to give a kind of “solid” sulfuric acid. Thecations are then conducted from sulfonic acid group to sulfonic acidgroup in a kind of hopping transport. Protons can be conducted bytunnelling or hopping particularly efficiently through the Nafion.Divalent cations are more likely to get stuck and not be transported anyfurther. Reference is therefore also made to polymer ion exchangers.

An example of a structural formula of sulfonatedpolytetrafluoroethylene:

The cause of the formation of gaseous carbon dioxide is attributable toneutralization of the hydronium ions that pass through by means ofcarbonate or hydrogencarbonate ions that are present. A strongly acidicelectrolyte, for example a strongly acidic anolyte, can additionallyenhance this effect: in the example that the anode space serves as aproton reservoir, an elevated proton pressure on the membrane isgenerated from the anode side and amplifies the concentration gradientin the cathode space. In this example, the anolyte, as described, maycomprise a Brønsted acid, for example sulfuric acid, phosphoric acid ornitric acid.

In the catholyte may be alkali metal or ammonium ions orhydrogencarbonates or carbonates. In the course of the carbon dioxidereduction, the starting composition of the catholyte, especially thehydrogencarbonate or carbonate concentration thereof, can be restoredvia the introduction or dissolution of carbon dioxide. An operation ofthis kind can be implemented, for example, as described, by theadditional use of a gas diffusion electrode.

In some embodiments, there is a proton-donating cathode in anarrangement for carbon dioxide reduction, which enables conversion ofthe hydrogencarbonate and carbonate ions present in the electrolyte tocarbon dioxide. By the methods described, it is possible to get aroundthe limitation of solubility of gaseous carbon dioxide in the immediateproximity of the reactive sites. Since only neutral carbon dioxide iselectrochemically reducible, and carbonate and hydrogencarbonate thatare the chemical equilibrium species thereof are not, this approachallows increasing the conversion of matter and hence also achieving highcurrent densities. In addition, it is thus possible to avoid oradditionally assist pressurization of the system as has been undertakento date, for example, for increasing the carbon dioxide saturation.

The process-intensifying method that has been presented for theelectrochemical reduction of carbon dioxide may improve the conversionof matter per unit electrode area and per unit current density. At thesame time, undesirably high carbonate and hydrogencarbonateconcentrations in the electrolyte, especially in the catholyte, areavoided, these having an adverse effect on the physical solubility ofthe carbon dioxide. The principle of a gas diffusion electrodeestablished in industry can be replaced by the method described. The gasdiffusion electrode can, however, further be used as an add-on to thisnew principle described, for example for the replenishment of carbondioxide in the electrolyte circuit. The method is particularly suitablefor use in electrolysis cells with external carbon dioxide saturation.

Some embodiments include the workup of the potassium hydrogencarbonatesolution obtained in basic carbon dioxide potash scrubbing within thescope of an in situ electrochemical regeneration of the laden absorbent.Compared to conventional thermal regeneration, the method offersenormous energy-saving potential.

The Hagg diagram shown in FIG. 1 contains values for a 0.05 molarsolution of carbon dioxide in water: the concentration of C in the unitmol/L is plotted against the pH. The proton concentration (H⁺), startingfrom a pH>0, drops from 1 to a value of 10⁻¹⁰ mol/L at a pH of 10, whilethe OH⁻ ion concentration rises in accordance with the definition of pH.Thus, while there is still a virtually pH-independent carbon dioxideconcentration (CO₂) of 0.05 mol/L in an acidic medium, i.e. up to a pHof about 4, this drops significantly starting from a pH of 5, in favorof a rise in the hydrogencarbonate ions (HCO₃ ⁻), which have theirhighest concentration within a pH range between 8 and 9. In a basicmedium at very high pH values >12, carbon dioxide is predominantly inthe form of carbonate ions (CO₃ ²⁻) in the solution.

The standard setups of electrolysis cells 2, 3, 4 shown in schematicform in FIGS. 2 to 4 comprise at least one anode A in an anode space ARand a cathode K in a cathode space KR. In each of the cases, the anodespace AR and cathode space KR are separated from one another at least bya membrane M₁. This membrane M1 may separate the gaseous products G1 andproducts P1, and/or prevent mixing. A defining parameter for themembrane M1 is what is called the bubble point. This describes thepressure difference AP between the two sides of the membrane M1 fromwhich gas flow would take place through the membrane M1. Thus, someembodiments may include a membrane M1 having a high bubble point of 10mbar or higher. The membrane M₁ here may be an ion-conducting membrane,for example an anion-conducting membrane or a cation-conductingmembrane. The membrane may be a porous layer or a diaphragm.

Lastly, the membrane M1 may also be understood to mean an ion-conductingspatial separator that separates electrolytes into anode space andcathode space AR, KR. According to the electrolyte solution E used, asetup without a membrane M₁ would also be conceivable. Anode A andcathode K are each connected electrically to a voltage supply. The anodespace AR of each of the electrolysis cells 2, 3, 4 shown is equippedwith an electrolyte inlet 21, 31, 41. Likewise, each anode space ARdepicted comprises an electrolyte outlet 23, 33, 43, via which theelectrolyte E and electrolysis products G1 formed at the anode A, forexample oxygen gas 02, can flow out of the anode space AR. Therespective cathode spaces KR each have at least one electrolyte outletand product outlet 24, 34, 44. The overall electrolysis product P1 heremay be composed of a multitude of electrolysis products.

While anode A and cathode K in the two-chamber setup 2 are in anarrangement separated from the membrane M₁ by the anode space AR andcathode space KR, the electrodes in what is called a polymer electrolytemembrane (PEM) setup 4 with porous electrodes directly adjoin themembrane M₁. As shown in FIG. 4, the anode may comprise a porous anode Aand the cathode may comprise a porous cathode K. In the two-chambersetup 2 and in the PET setup 4, the electrolyte and the carbon dioxideCO₂ may be introduced into the cathode space KR via a common reactantinlet 22, 42.

In a different manner, as shown in FIG. 3, in what is called athree-chamber setup 3, in which the cathode space KR has an electrolyteinlet 32, the carbon dioxide CO₂ is fed into the cathode space KRseparately therefrom via the cathode K, which in this case isnecessarily in porous form. In some embodiments, the porous cathode Kcomprises a gas diffusion electrode GDE. A gas diffusion electrode GDEis characterized in that the liquid component, for example anelectrolyte, and a gaseous component, for example an electrolysisproduct, can be contacted with one another in a pore system of theelectrode, for example the cathode K. The pore system of the electrodeis in such a form that the liquid and gaseous phase alike can penetrateinto the pore system and be present simultaneously therein. Typically,for this purpose, a reaction catalyst is in porous form and assumes theelectrode function, or a porous electrode includes catalytically activecomponents. For introduction of the carbon dioxide CO₂ into thecatholyte circuit, the gas diffusion electrode GDE comprises a carbondioxide inlet 320.

It would be possible to implement the teachings herein in one of theelectrolysis cell setups known to date, as shown, for example, in FIGS.2 and 3, if they were provided with an appropriate proton donor unit.The setup shown in FIG. 4 would require more specific modifications forthe implementation, for example transport channels for the electrolytethrough the cathode, in order to establish membrane-electrolyte contact.In some embodiments, in these transport channels, carbon dioxideevolution or release would take place. Analogously, on the anode side,transport channels for the analyte to the membrane are required in orderthus to provide the protons.

By means of a polymer electrolyte setup of this kind with porouselectrodes having transport channels, it is possible to implement a casein which almost exactly as much carbon dioxide is produced as is thenalso reduced at the cathode. In this way, by contrast with the gasdiffusion electrode, as known to date, it is possible to arrive atparticularly highly enriched products. The electrolysis cells known fromthe prior art can also be modified for the use in such a way that theyare combined to give mixed variants. For example, an anode space may beexecuted as a polymer electrolyte membrane half-cell, whereas a cathodespace consists of a half-cell, with cathode space between membrane andcathode, as shown in FIGS. 2 and 3.

FIG. 5 shows, in schematic form, the setup of an electrolysis cell 5with an anode space AR between an anode A and a membrane M1, and acathode space KR between the membrane M1 and the cathode K. Anode A andcathode K are connected to one another via a voltage supply. An arrowfrom the anode space AR into the cathode space KR through the membraneM1 indicates that it is ion-conducting at least to one type of chargecarrier, e.g. at least to cations X⁺, where these can be different metalcations X⁺ depending on which anolyte is used, and to protons H⁺. Thecathode space KR has a width d_(MX), i.e. a distance between membrane M1and cathode K. The membrane M1 and the cathode K are arranged in theelectrolysis cell 5 such that the surfaces thereof facing the cathodespace KR run plane-parallel to one another. A slope triangle indicatesthe pH gradient in the cathode space KR: the pH rises from a locallyacidic environment close to the membrane M1 to a locally basicenvironment close to the cathode surface K. The locally acidic region isidentified by I and represented by a dotted line parallel to M1;correspondingly, II and the dotted line in front of the cathode K showthe region which is locally basic in the cathode space KR. The anodespace AR becomes acidic to the same degree as the cathode space KRbecomes basic.

Another look at the equilibrium reaction 1 explains the pH gradient asfollows: the anions and cations that are present and form on thedifferent sides of the membrane M1 can migrate, within the electrolyte Eand through the membrane M1. The electrons provided at the anode A, forexample in an aqueous electrolyte E, convert the water to H⁺ ions andoxygen gas O₂. If carbon dioxide CO₂ is, for example, in chemicallybound form as hydrogencarbonate HCO₃ ⁻ in the anolyte and/or catholyte,it can react further with the protons H⁺ to give carbon dioxide gas CO₂and water H₂O. The catholyte preferably comprises alkali metal and/orammonium ions or the hydrogencarbonates or carbonates thereof. Thereaction of hydrogencarbonate HCO₃ ⁻ to give carbon dioxide CO2 isreferred to as the acidic breakdown of hydrogencarbonate HCO₃ ⁻. In abasic medium, i.e. at a pH between 6 and 9, hydrogencarbonate HCO₃ ⁻ isformed, meaning that the equilibrium reaction eq. 1 then runs the otherway. Thus, if a potassium hydrogencarbonate solution, for example, isthen used as anolyte and as catholyte in an electrolysis cell 5, the pHgradient shown in FIG. 5 from a locally acidic environment I forms inthe phase boundary layer between M1 and catholyte, in which the carbondioxide is preferentially released. At the cathode surface or in thephase boundary layer II between the cathode surface and catholyte,however, the pH, owing to ion migration, is already sufficiently highagain, for example within a range between 6 and 9, that the reaction ofpotassium hydrogencarbonate formation predominates and hence only littlephysically bound carbon dioxide CO₂ is available in the electrolytesolution E for reduction at the cathode K.

The distance d_(MX) accordingly has to be chosen at such a minimum levelthat the phase interface layer I between the membrane M1 and catholytethat functions as the carbon dioxide source abuts, or overlaps orcoincides with, the phase interface layer II between the cathode surfaceK and catholyte, such that sufficient released carbon dioxide CO₂ isprovided or replenished at the reaction interface of the cathode K.

FIGS. 6 to 9 show various embodiments of electrolysis cells. These arein principle designed according to the polymer electrolyte membrane(PEM) setup, or polymer electrolyte membrane half-cell setup. Firstly,the proton-donating membrane of the proton donor unit can be arranged inthe immediate proximity of the cathode, as in FIGS. 7 and 9; secondly,the cathode can be integrated into the proton donor unit with theproton-donating membrane, as shown by way of example by FIGS. 6 and 8.

The polymer electrolyte membrane (PEM) is frequently also called protonexchange membrane and is a semipermeable membrane. These membranes arepreferably permeable to cations such as protons H⁺, lithium cations L⁺,sodium cations Na⁺ or potassium cations K⁺, while the transport ofgases, for example oxygen O₂ or hydrogen H₂, is prevented. This purposeis fulfilled by the membrane M₁, for example in the separation of theproducts P1, G1 of the anode and cathode reactions. In most cases,aqueous liquids can flow through the PEM, although the capillary forcesinhibit this transport. A polymer electrolyte membrane may be produced,for example, from an ionomer, pure polymer membranes or compositemembranes, wherein other materials are embedded into a polymer matrix.One example of a commercially available polymer electrolyte membrane isNafion from DuPont.

All setups have the same sequence of, on the left-hand side, an anodespace AR separated from the cathode space KR by an anode A and amembrane M1 abutting the side of the anode A facing away from the anodespace AR. The cathode space KR is abutted by the cathode K, and thelatter by the proton donor unit in different designs. Arrows indicatethe reactant and electrolyte inlets E into the anode space AR andcathode space KR, and the outlets for electrolyte mixtures E and productmixtures P1, G1. The membrane M1 serves predominantly as separatormembrane, but may also be proton-permeable, as required, for example,for the embodiment with an additional acid reservoir PR1 on the anodeside. The acid or proton reservoir PR on the cathode side is dividedfrom the cathode K in all cases by a proton-conducting membrane M2. InFIGS. 7 and 9, the cathode K is between two catholyte gaps KS orintegrated into the proton donor unit as a proton donor cathode PSK. Inthe cases of the cell arrangement as shown in FIGS. 6 and 8, the porouscathode K is not just in proton-permeable form, but preferably alsoelectrolyte-permeable form, such that the carbon dioxide release canoccur over a very large cathode surface area, for example withinelectrolyte channels, in the cathode K. In the examples with solidcathode K and a very narrow cathode space, which should instead bereferred to as catholyte gap KS, owing to its small extent between thereactive area and the membrane surface M2, via which the protons H⁺ arepreferably supplied, the cathode K may be formed from a solid metalsheet, but may also have advantageous nanostructuring to increase thesurface area. In the case of division of the cathode space KR into twocatholyte gaps KS, as shown in FIGS. 7 and 9, the acid flowing past thecathode K can form the anolyte, since protons H⁺ are then generated onthe anode side by water oxidation, for example. In FIGS. 8 and 9, theanode space AR is explicitly designed as an additional proton reservoirPR1 and the anolyte used is an acid. In these cases, the two proton oracid reservoirs PR, PR1 may be connected to one another via acirculation system. The catholyte gaps KS shown in FIGS. 7 and 9 have awidth, for example, between 0 and 5 mm, advantageously between 0.1 and 1mm, preferably between 0.1 and 0.5 mm. In the cases shown in FIGS. 8 and9, the separator membrane M1 may be in proton-conducting form; at leastone membrane M1 that ensures charge balance is used.

For the cases shown in FIGS. 6 and 9 with proton donor cathode PSK andcathode space KR, however, it is important not to arbitrarily constrictthe cathode space KR and to reduce it to a cathode gap KS: in thesecases, a minimum distance b_(KR) is actually necessary between separatormembrane M1 and proton donor cathode PSK, since the hydronium ions thatpass through, given too small an electrolyte volume between separatormembrane M1 and cathode K, would otherwise be converted primarily tohydrogen H2 at the catalyst interface, and hence no carbon dioxidereduction could take place. The protons H⁺ that enter the cathode spaceKR must firstly ensure the carbon dioxide formation and must not beconverted directly to hydrogen H₂. The minimum distance b_(KR) for thecathode space KR in the case of a cell arrangement 6, 8 with protondonor cathode PSK is 1 mm. In some embodiments, the distance b_(KR)between separator membrane surface M1 and catalyst surface K is between1 and 10 mm, not more than 5 mm, or not more than 2 mm.

In some embodiments, the absolute carbon dioxide concentration of theliquid phase, but in particular the local availability of the physicallydissolved carbon dioxide in the immediate proximity of the electrodesurface. Macrokinetic mass transfer operations play only a minor role inthe arrangement of the invention, since the carbon dioxide required forelectrochemical reduction is effectively provided from the anions of theelectrolyte by in situ protonation at the reaction surface.

What is claimed is:
 1. An electrolysis system for carbon dioxideutilization, the system comprising: an electrolysis cell with an anodein an anode space, a cathode in a cathode space, and a membrane; a firstfeed for carbon dioxide into the cathode space, configured to bring thecarbon dioxide into contact with the cathode; a proton donor unit; and asecond feed for protons configured to bring the protons into the cathodespace from the proton donor unit into contact with the cathode.
 2. Theelectrolysis system as claimed in claim 1, wherein: the proton donorunit comprises a proton reservoir; and the second feed comprises aproton-permeable membrane.
 3. The electrolysis system as claimed inclaim 1, wherein the proton reservoir comprises an acid reservoirholding a Brønsted acid.
 4. The electrolysis system as claimed in claim2, wherein the proton-permeable membrane comprises sulfonatedpolytetrafluoroethylene.
 5. The electrolysis system as claimed in claim1, wherein: the cathode space includes a catholyte/carbon dioxidemixture; and the catholyte comprises at least one of carbonate,hydrogencarbonate anions, or dihydrogen carbonate.
 6. The electrolysissystem as claimed in claim 1, wherein the anode space includes a protonreservoir.
 7. The electrolysis system as claimed in claim 1, furthercomprising a first membrane and a second membrane; wherein the firstmembrane is arranged between the anode and cathode; the second membraneis arranged between the cathode and proton reservoir; and the secondmembrane is proton-permeable.
 8. The electrolysis system as claimed inclaim 1, wherein the cathode space comprises a catholyte gap extendingalong the cathode; Wherein the catholyte gap has an extent at rightangles to a surface area of the cathode of not more than 5 mm.
 9. Theelectrolysis system as claimed in claim 1, wherein the cathode spacecomprises a catholyte gap separating the cathode and membrane; whereinthe cathode and the membrane are arranged at a distance of not more than5 mm from one another.
 10. The electrolysis system as claimed in claim1, wherein the cathode space comprises two catholyte gaps arranged oneither side of the cathode, each of which is bounded by a membrane;wherein the cathode and membranes are each independently arranged at amaximum distance of 5 mm from one another.
 11. The electrolysis systemas claimed in claim 1, wherein the proton donor unit comprises a protondonor cathode and a proton-permeable cathode integrated therein.
 12. Areduction method for carbon dioxide utilization, the method comprising:introducing a catholyte/carbon dioxide mixture into a cathode space andinto contact with a cathode disposed in the cathode space; and loweringa pH of the catholyte/carbon dioxide mixture in the cathode space byproviding additional protons from a proton donor unit.
 13. The reductionmethod as claimed in claim 12, wherein lowering the pH of thecatholyte/carbon dioxide mixture occurs at a liquid/solid phaseinterface from the catholyte/carbon dioxide mixture to the cathode; andfurther comprising providing the protons via a proton-permeable membraneor a proton-permeable cathode at a liquid/solid phase interface from thecatholyte/carbon dioxide mixture to the cathode.
 14. The reductionmethod as claimed in claim 12, further comprising supplying the protonsfrom a proton reservoir comprising an acid reservoir including aBrønsted acid (HX).
 15. The reduction method as claimed in claim 12,wherein the catholyte includes carbonate and/or hydrogencarbonateanions.